The Empress - it gets shortened to 'Hempress'. Colney Hatch was apparently the name of an insane asylum in London. So this reference infers that someone is the 'empress of the mad', so no-one important at all!

Just to add to Rachel's explanation... there are a couple of dialect notes here. A number of British dialects and accents often drop the H. The original text should have an apostrophe in front: 'Atch (Hatch). The effect for the listener is that the person probably comes from the countryside. On the other hand, adding H to words beginning with vowel sounds sounds comically pompous: Hempress (Empress). So, in just a few words the tone has moved from exceedingly formal (Hempress) to very colloquial ('Atch). Nice little effect, huh?

The Magician's Nephew - a great book!